AS the other guys have said, keep it up, as it seems the more they reply, the more they are putting thier foot in it.
The more this goes round incircles with no real facts, the stronger your case will be when it comes to getting your full deposit back from the DPS, as you have been reasonable and conciencious all along, your communications show this.
Remember the DPS is independant, thier purpose is to arbitrate deposit disputes without the need for small claims court.
Afterall, as you said, if indeed they have a figure for this so-called damage, then they would have no problem proving to you how they arrived at said figure.
It all points to typical letting agency tricks, normaly these things dont get contested because the tennant has no confidence of success, and they get to walk away with the cash.